Method for controlling a wind turbine by optimizing its production while minimizing the mechanical impact on the transmission

ABSTRACT

A method is disclosed for controlling a wind turbine by optimizing its production while minimizing the mechanical impact on the transmission. The wind turbine comprises a nacelle provided with a rotor on which blades are fastened, and an electrical machine linked to the rotor by a transmission, in which an pitch angle of the blades is controlled, comprising: An aerodynamic torque setpoint and an electrical machine torque setpoint making possible maximizing the recovered power are determined, from measurements of wind speed, of rotor speed and of electric machine speed. At least one of the setpoints is modified by subtracting from it a term proportional to a difference between the measured speed of the rotor and the measured speed of the electric machine. A pitch angle of the blades making possible production of the aerodynamic torque setpoint is determined. The blades are oriented according to the angle of inclination.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of renewable energies andmore particularly to the control of wind turbines.

2. Description of the Prior Art

A wind turbine makes possible transformation of the kinetic energy ofthe wind into electrical or mechanical energy. The wind turbine has thefollowing elements:

A mast used to place the rotor at a sufficient height to allow for itsmovement (necessary for the wind turbines with a horizontal axis) or toplace the rotor at a height that allows it to be driven by a wind thatis stronger and more regular than at ground level. The mast generallyhouses some of the electrical and electronic components (modulator,control, multiplying year, generator, etc.).

A nacelle mounted at the top of the mast, housing the mechanical andpneumatic components, and some of the electrical and electroniccomponents, necessary to the operation of the machine. The nacelle canrotate to orient the machine in the right direction.

A rotor, having blades (generally three) and the nose of the windturbine fastened to the nacelle. The rotor is driven by the energy ofthe wind and is linked by a mechanical shaft directly or indirectly (viaa mechanical gearbox and shaft system) to the electrical machine(electrical generator, etc) which converts the energy collected intoelectrical energy.

A transmission, with two axes (mechanical shaft of the rotor andmechanical shaft of the electrical machine) linked by a gearbox.

In the case of offshore wind, a distinction is made between the casewhere the wind turbine is placed on the seabed (fixed or establishedwind turbine), and the case where the wind turbine is supported by aplatform which floats on the sea and which is anchored to the seabed(floating wind turbine).

Since the beginning of the 1990s, there has been an upsurge of interestin wind energy, in particular in the European Union where the annualgrowth rate is approximately 20%. This growth is attributed to theproduction of electricity without carbon emissions. In order to sustainthis growth, the efficiency of the wind turbines has to continue to beimproved. Wind turbines are designed to produce electricity at a pricethat is as low as possible. Consequently, the wind turbines aregenerally constructed to achieve maximum performance at approximately 15m/s. It is in fact pointless to design wind turbines which maximizetheir efficiency at even higher wind speeds, since such speeds areinfrequent. In the case of wind speeds greater than 15 m/s, it isnecessary to lose a portion of the additional energy contained in thewind in order to avoid any damage to the wind turbine. All the windturbines are therefore designed with a power regulation system.

Increasing wind energy production requires developing effectiveproduction tools and sophisticated control tools to enhance theperformance levels of the machines. Consequently, the wind turbines aregenerally constructed to achieve their maximum performance atapproximately 15 m/s.

Linear controllers have been widely used for the power regulation bycontrolling the pitch angle of the blades (orientation of the blades).Techniques that use PI and PID controllers, LQ and LQG controltechniques and strategies based on robust linear controls are known.

However, the performance levels of these linear controllers are limitedby the greatly non-linear characteristics of the wind turbine. Firststrategies based on non-linear controls were used in: Boukhezzar B.,Lupu L., Siguerdidjane H., Hand M. “Multivariable Control Strategy forVariable Speed, Variable Pitch Wind Turbines” Renewable Energy, 32(2007)1273-1287.

However, none of these controllers makes it possible to take account forthe mechanical impact (fatigue and extreme moment) on the transmission.Most wind turbine failures are due to breakages or damage affecting thetransmission. From data recovered on an offshore application, breakagesof the transmission, of the gearbox or of the electrical machinerepresent nearly 39% of the time when the wind turbine is not producing.

SUMMARY OF THE INVENTION

The invention relates to a method for optimizing the electrical energyproduction of a wind turbine, by implementing a non-linear control ofthe orientation of the blades that accounts for the dynamics of thesystem, while minimizing the mechanical impact on the transmission. Theimpact is minimized by reducing the torsion speed variations of thetransmission by accounting for the drift of the torsion angle of thetransmission.

Generally, the invention relates to a method for optimizing theelectrical energy production of a wind turbine comprising a nacelleprovided with a rotor on which blades are fastened, and an electricalmachine linked to the rotor by a transmission, in which a pitch angle ofthe blades is controlled. The method comprises:

a) determining an aerodynamic torque setpoint and an electric machinetorque setpoint which maximizes the recovered power, from measurementsof wind speed, of rotor speed and of the electrical machine speed;

b) at least one of the setpoints is modified by subtracting from it aterm proportional to a difference between the measured speed of therotor and the measured speed of the electrical machine;

c) determining a pitch angle of the blades that produces aerodynamictorque setpoint; and

d) orienting the blades according to the angle of inclination.

According to the invention, at least one of the setpoints is modified bycarrying out the following steps:

i) a torque T _(res) on the transmission resulting from the aerodynamictorque and electric machine torque setpoints is determined;

ii) a resultant torque setpoint T_(res) ^(sp) is determined bysubtracting from the resultant torque T _(res) a term proportional tothe difference between the measured speed of the rotor and the measuredspeed of the electric machine;

iii) the aerodynamic torque setpoint is modified by dividing up theresultant torque setpoint into an aerodynamic torque and an electricmachine torque. According to the invention, the resultant torquesetpoint T_(res) ^(sp) can be expressed as follows:

T _(res) ^(sp) = T _(res) −ký _(tr)

k is being strictly positive calibration parameters, and ý_(tr) is thespeed of the torsion of the transmission, equal to a difference in speedof the rotor Ω_(r), and of the electrical machine Ω_(g) related to thesame axis:

${{\overset{.}{\gamma}}_{tr} = {\Omega_{r} - {\frac{1}{N}\Omega_{g}}}},$

where N is a gear ratio between the axis of the rotor and the axis ofthe electrical machine.

The pitch angle of the blades can be determined by inverting anaerodynamic torque model and by using the wind speed and rotor speedmeasurements.

Finally, the proportional term can be determined by using a model of thedynamics of the transmission.

Other features and advantages of the method according to the inventionwill become apparent on reading the following description ofnon-limiting exemplary embodiments, by referring to the appendeddrawings described hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents the sequencing of the steps of the method according tothe invention.

FIG. 2 illustrates an exemplary mapping of the parameter c_(q).

DETAILED DESCRIPTION OF THE INVENTION

In the description, the following notations are used:

Controlled variables:

-   θ is the pitch angle of the blades in degrees, which is also called    “pitch” and which corresponds to the angle of attack to the wind of    each of the blades.-   T_(g) is the torque of the electric machine in Nm; and-   T_(aero) is the aerodynamic torque (rotational force applied to the    rotor under the effect of the wind).

Measured variables, denoted MEAS(-):

-   V_(W) is the speed of the wind in m/s which is derived from a    measurement from an anemometer or derived from an estimation;-   Ω_(r) is the speed of the rotor in rad/s; and-   Ω_(g) the speed of the electric machine in rad/s.

The method according to the invention maximizes the energy production ofan onshore or offshore wind turbine while limiting the extreme momentsand the fatigue of the transmission. To do this, a rotor speed setpointand an electrical machine torque setpoint are determined first tomaximize the recovered power. These two setpoints are obtained bymappings which are a function of the wind speed. This type of mapping iswell known.

To control the mechanical structure, the aerodynamic torque applied tothe hub is driven by actuating the orientation of the blades. To dothis, models are used of this average aerodynamic torque as a functionof the pitch, of the wind speed and of the speed of the rotor. Then, theposition setpoint of the pitch and of the torque of the electric machineis modified in order to limit the mechanical impact of the windvariations. Thus, to model driving the system, the steps illustrated inFIG. 1 are carried out:

E1. Determination of the pitch making possible optimizing the recoveredpower

-   -   i. Generation of an electrical torque setpoint T_(g) ^(sp)    -   ii. Generation of an aerodynamic torque setpoint T_(aero) ^(sp)    -   iii. Determination of a pitch position θ

E2. Determination of the resultant torque of the torque setpoint T_(g)^(sp) and T_(aero) ^(sp)

E3. Generation of a resultant torque setpoint (T_(res) ^(sp)) whichreduces the fatigue and the extreme moments of the transmission

E4. Dividing up of the resultant torque setpoint (T_(res) ^(sp)) betweenthe aerodynamic and electrical torques

E5. Determination of a pitch position which makes it possible to producethis aerodynamic torque

E6. Orientation of the blades according to the determined pitch

1. Determination of the Pitch Making it Possible to Optimize theRecovered Power

One aspect of the method according to the invention is to maximize theenergy production of a wind turbine with a horizontal axis (propeller atright angles to the wind), installed onshore or offshore, while limitingthe extreme moments and the fatigue of the mechanical structure.

To maximize the energy production of a wind turbine, the pitch angle ofthe blades, called “pitch” and denoted θ, that makes it possible tomaximize the recovered power P_(aero) as a function of the wind speedV_(W), is sought. The orientation of the blades is the angle between theblades and a reference such as the ground (horizontal plane, at rightangles to the mast of the wind turbine).

According to one embodiment, to define this angle, a model of therecoverable power is used. This power P_(aero) can be expressed:

P _(aero) =T _(aero)*Ω_(r)

with:

-   -   T_(aero) being the aerodynamic torque (rotational force applied        to the rotor under the effect of the wind);    -   Ω_(r) being the speed of the rotor in rad/s.

The angle θ which makes it possible to maximize P_(aero) is thereforesought. To do this, the following steps are carried out:

-   -   i.—Generation of a torque setpoint of the electric machine T_(g)        ^(sp)    -   ii.—Generation of an aerodynamic torque setpoint T_(aero) ^(sp)    -   iii.—Determination of a pitch position θ        i—Generation of a Torque Setpoint of the Electric Machine T_(g)        ^(sp)

A torque setpoint of the electric machine T_(g) ^(sp) is firstdetermined. This setpoint is obtained mapping as a function of the speedof the electric machine.

According to the invention, the aerodynamic torque T_(aero) is modeledby a model describing the power of the wind contained in a cylinder,multiplied by a factor describing the fact that a wind turbine allowsonly a portion of this power to be recovered. The aerodynamic torque isthus modeled as a function of the speed of the wind V_(W), of the pitchθ and of the speed of the rotor Ω_(r). Such a model can thus beexpressed, in steady state operation:

$\begin{matrix}{T_{aero} = {0.5\; \rho \; \Pi \; R_{b}^{3}{c_{q}( {\theta,\frac{R_{b}\Omega_{r}}{V_{w}}} )}V_{w}^{2}}} & (1)\end{matrix}$

With:

-   -   R_(b): the radius of the rotor;    -   ρ: the density of the air;    -   c_(q): the mapping to be calibrated.

An exemplary mapping of the parameter c_(q) is presented in FIG. 2. Thismapping indicates the value of the parameter c_(q) as a function of theratio

$\frac{R_{b}\Omega_{r}}{V_{w}}$

for different pitches (one curve for each θ). This type of mapping iswell known to the experts. The ratio

$\frac{R_{b}\Omega_{r}}{V_{w}}$

is denoted TSR in FIG. 2.

Thus, to determine the torque setpoint of the electric machine as afunction of the speed of the electric machine, the recovered aerodynamicpower is optimized for each wind speed.

$T_{g}^{sp} = {\arg ( {\max_{\theta,V_{w}}{\frac{0.5}{N}\rho \; \Pi \; R_{b}^{3}{c_{q}( {\theta,\frac{R_{b}\Omega_{g}}{{NV}_{w}}} )}V_{w}^{2}}} )}$

This gives us the setpoint torque Tg which depends on the speed of theelectric machine: T_(g) ^(sp)=f(Ω_(g))

However, compared to this reference curve, two limitations are applied:

-   -   a zero torque for the low speeds of the electrical machine to be        able to increase the speed of the wind turbine;    -   a maximum torque to limit the power of the electrical machine.

Thus, there are three regions on the curve T_(g) ^(sp)=f(Ω_(g)):

-   -   Region 1: zero torque;    -   Region 2: optimum torque;    -   Region 3: torque limited by the maximum power.        ii—Generation of an Aerodynamic Torque Setpoint T_(aero) ^(sp)

The objective is to generate an aerodynamic torque setpoint T_(aero)^(sp) which makes it possible to produce the setpoint rotor speed Ω_(g)^(sp). For this, a model of the dynamics of the rotor is used.

${J_{r}\frac{\Omega_{r}}{t}} = {T_{aero} - {T_{l}( \Omega_{r} )} - {{NT}_{g}( \Omega_{g} )}}$

with:

-   -   J_(r) being inertia of the rotor;    -   T_(I)(Ω_(r)) being friction and load torque on the rotor (a        second order polynomial is conventionally used);    -   N being a gear ratio between the axis of the rotor and the axis        of the electric machine.

Thus, the control strategy used is a dynamic control strategy whichanticipates the setpoint variation and which corrects with two terms, aproportional term and an integral term. The strategy is expressed:

$T_{aero}^{sp} = {{T_{l}( \Omega_{r} )} + {{nT}_{e}( \Omega_{r} )} + {J_{r}\frac{\Omega_{r}^{sp}}{t}} - {k_{p}( {\Omega_{r} - \Omega_{r}^{sp}} )} - {k_{i}{\int( {\Omega_{r} - \Omega_{r}^{sp}} )}}}$

where kp and ki are two real parameters to be calibrated to guaranteethe convergence of the speed toward its setpoint.

iii—Determination of a Pitch Position θ

From this aerodynamic torque setpoint T_(aero) ^(sp), a pitch angle θ ofthe blades is determined which satisfies this aerodynamic torque demandT_(aero) ^(sp). For this, the aerodynamic torque model (equation 1) isused, with the measurement of the speed of the wind V_(W), themeasurement of the speed of the rotor Ω_(r) ^(sp), and the setpointtorque T_(aero) ^(sp). By inverting the model (by a Newton algorithm forexample), a pitch setpoint θ is obtained:

$\overset{\_}{\theta} = {\arg ( {\min_{\theta}( {T_{aero}^{sp} - {0.5\; \rho \; \Pi \; R_{b}^{3}{c_{q}( {\theta,\frac{R_{b}\Omega_{r}}{V_{w}}} )}V_{w}^{2}}} )^{2}} )}$

Thus, with this control law, the convergence toward the reference rotorspeed is guaranteed which makes it possible to maximize the recoveredpower.

2—Determination of the Resultant Torque of the Torque Setpoints T_(g)^(sp) and T_(aero) ^(sp)

From the setpoints T_(g) ^(sp) and T_(aero) ^(sp), the torque T _(res)resulting from these two torques and which will bear on the transmissionis determined. For this, this torque is modelled by the followingformula:

$\begin{matrix}{{\overset{\_}{T}}_{res} = {{\frac{J_{g}}{J_{r} + J_{g}}( {T_{aero}^{sp} - T_{l}} )} + {\frac{J_{r}}{J_{r} + J_{g}}{NT}_{g}^{sp}}}} & (2)\end{matrix}$

wherein J_(r) and J_(g) are the inertias of the rotor and of theelectric machine.

3—Generation of a Resultant Torque Setpoint (T_(res) ^(sp)) WhichReduces the Fatigue and the Moments of the Transmission

Efforts are made to modify this resultant torque T _(res) in order tominimize the impact on the transmission and therefore increase its life.For this, efforts are made to reduce the torsion speed variations of thetransmission. Thus, efforts are made to compensate the torque with termsproportional to the difference between the speed of the rotor and of theelectric machine. The dynamics of the mechanical structure (dynamics ofthe transmission) can be expressed in the form of two coupled secondorder systems.

$\begin{matrix}\{ \begin{matrix}{{\frac{J_{r}J_{g}}{J_{r} + J_{g}}{\overset{¨}{\gamma}}_{tr}} = {{{- c_{d}}\gamma_{tr}} - {k_{d}{\overset{.}{\gamma}}_{tr}} + {\frac{J_{g}}{J_{r} + J_{g}}( {T_{aero} - T_{l}} )} + {\frac{J_{r}}{J_{r} + J_{g}}{NT}_{g}}}} \\{{J_{g}{\overset{.}{\Omega}}_{g}} = {{c_{d}\gamma_{tr}} + {k_{d}{\overset{.}{\gamma}}_{tr}} + {N_{gb}T_{g}}}}\end{matrix}  & (3)\end{matrix}$

where

-   -   γ_(tr), {dot over (γ)}_(tr) and ÿ_(tr) are respectively the        angle, the speed and the acceleration of the torsion of the        shaft. It should be noted that the torsion speed of the        transmission is the difference in speed of the rotor and of the        generator related to the same axis, i.e.

${{\overset{.}{\gamma}}_{tr} = {\Omega_{r} - {\frac{1}{N}\Omega_{g}}}};$

-   -   kd is the structural damping of the transmission;    -   cd is the stiffness of the transmission.

Thus, the control strategy generates a resultant torque different from T_(res) to minimize the fatigue and the extreme moments of thetransmission. Therefore, the relationship:

T _(res) ^(sp)= T _(res) −k{dot over (γ)}_(tr)

with k being strictly positive calibration parameters. These parameterscan be determined by trial and error. It can be considered that allthese parameters k are equal to 1 for example.

4—Dividing Up of the Resultant Setpoint Torque (T_(res) ^(sp)) Betweenthe Aerodynamic and Electrical Torques

This resultant torque setpoint T_(res) ^(sp) is then divided up betweenthe aerodynamic torque T_(aero) and the torque of the electric machineT_(g). For this, the dividing up is done according to operational areas.In an area 2, where the aerodynamic torque is limiting, a reserve oftorque is present. In this case, the torque modification influences thetorque of the electrical machine and not the aerodynamic torque. Thus,in this case, the relationship:

$\begin{matrix}\{ \begin{matrix}{T_{aero}^{strat} = T_{aero}^{sp}} \\{T_{g}^{strat} = {T_{g}^{sp} - {k\frac{J_{r} + J_{g}}{{NJ}_{r}}{\overset{.}{\gamma}}_{tr}}}}\end{matrix}  & (4)\end{matrix}$

Similarly, in an area 3, where the torque of the electrical machine islimiting, the torque modification influences the aerodynamic torquewhich gives the relationship:

$\begin{matrix}\{ \begin{matrix}{T_{aero}^{strat} = {T_{aero}^{sp} - {k\frac{J_{r} + J_{g}}{J_{g}}{\overset{.}{\gamma}}_{tr}}}} \\{T_{g}^{strat} = T_{g}^{sp}}\end{matrix}  & (5)\end{matrix}$

5—Determination of a Pitch Position Makes Possible Production OtherAerodynamic Torque

From the aerodynamic torque setpoint T_(aero) ^(strat), a pitch angleθ^(SP) of the blades is determined to satisfy this aerodynamic torquedemand T_(aero) ^(strat). For this, the aerodynamic torque model(equation 1) is used, with the measurement of the speed of the windV_(W), the measurement of the speed of the rotor Ω_(r) ^(sp), and thesetpoint torque T_(aero) ^(strat). By inverting the model (by a Newtonalgorithm for example), a pitch setpoint θ^(SP) is obtained:

$\theta^{SP} = {\arg ( {\min_{\theta}( {T_{aero}^{strat} - {0.5\; \rho \; \Pi \; R_{b}^{3}{c_{q}( {\theta,\frac{R_{b}\Omega_{r}}{V_{w}}} )}V_{w}^{2}}} )^{2}} )}$

Thus, with this control law, the convergence toward the reference rotorspeed is guaranteed, making it possible to maximize the recovered power,while minimizing the mechanical impact (fatigue and extreme moment) onthe transmission.

6—Orientation of the Blades According to the Determined Pitch

To optimize the electrical power recovered by the wind turbine, theblades are oriented according to the pitch angle calculated in thepreceding step.

1-5. (canceled)
 6. A method for optimizing electrical energy productionof a wind turbine, the wind turbine comprising a nacelle provided with arotor on which blades are fastened, and an electrical machine linked tothe rotor by a transmission, in which an pitch angle of the blades iscontrolled, comprising: a) determining an aerodynamic torque setpointand an electric machine torque setpoint that make possible maximizingrecovered power, from measurements of wind speed, of rotor speed and ofelectric machine speed; b) modifying at least one of the setpoints bysubtracting a term proportional to a difference between the measuredspeed of the rotor and the measured speed of the electric machine; c)determining a pitch angle of the blades making possible production ofthe aerodynamic torque setpoint; and d) orienting the blades accordingto the angle of inclination.
 7. A method according to claim 1, whereinat least one of the setpoints is modified by: i) determining a torque T_(res) on the transmission resulting from the aerodynamic torque andelectrical machine torque setpoints; ii) determining a resultant torquesetpoint T_(res) ^(sp) by subtracting from the resultant torque T _(res)a term proportional to a difference between the measured speed of therotor and the measured speed of the electric machine; and iii) modifyingaerodynamic torque setpoint by dividing up the resultant torque setpointinto an aerodynamic torque and an electrical machine torque.
 8. A methodaccording to claim 7, in which said resultant torque setpoint T_(res)^(sp) is expressed as:T _(res) ^(sp) = T _(res) −k{dot over (y)} _(tr) with k being strictlypositive calibration parameters, and {dot over (y)}_(tr) being a speedof torsion of the transmission, equal to a difference in speed of therotor Ω_(r), and of the electrical machine Ω_(g) related to one axis:${{\overset{.}{\gamma}}_{tr} = {\Omega_{r} - {\frac{1}{N}\Omega_{g}}}},$where N is a gear ratio between the axis of the rotor and the axis ofthe electrical machine.
 9. A method according to claim 6, in which thepitch angle of the blades is determined by inverting an aerodynamictorque model while using the wind speed and rotor speed measurements.10. A method according to claim 7, in which the pitch angle of theblades is determined by inverting an aerodynamic torque model whileusing the wind speed and rotor speed measurements.
 11. A methodaccording to claim 8, in which the pitch angle of the blades isdetermined by inverting an aerodynamic torque model while using the windspeed and rotor speed measurements.
 12. A method according to claim 6,in which the proportional term is determined by using a model ofdynamics of the transmission.
 13. A method according to claim 7, inwhich the proportional term is determined by using a model of dynamicsof the transmission.
 14. A method according to claim 8, in which theproportional term is determined by using a model of dynamics of thetransmission.
 15. A method according to claim 9, in which theproportional term is determined by using a model of dynamics of thetransmission.
 16. A method according to claim 10, in which theproportional term is determined by using a model of dynamics of thetransmission.
 17. A method according to claim 11, in which theproportional term is determined by using a model of dynamics of thetransmission.